Helium II (He II) refers to the second liquid phase of the most abundant helium isotope (4He). Helium II is also referred to as superfluid helium. Helium II occurs once the temperature of the liquid helium drops below 2.17K.
The phase transition between the first liquid phase of Helium (referred to as Helium I) and Helium II is a second order phase transition, which in practical terms means that there is no latent heat required for the transition. At this transition, however, there is a discontinuity in the specific heat. This discontinuity as a function of temperature takes the shape of the Greek letter lambda. Thus, the transition is known as the lambda transition and 2.17K is referred to as the lambda temperature.
Helium II results because a proportion of the helium has condensed into the ground quantum state and possesses zero viscosity and zero entropy. The remaining fraction of the helium is in the excited quantum states and retains finite viscosity and entropy. It is the presence and interaction of these two fractions that produce the unique properties of He II and these properties can be described by the Two Fluid Model.
Properties of Helium II include extremely effective heat transfer and under certain circumstances zero viscosity and unique film flow behavior. Arguably, the most valuable property of He II from an application standpoint is simply its lower temperature.
Going below 2K permits higher magnet fields for NbTi superconductors, and better performance of superconducting radio frequency (SCRF) cavities. Additionally, cooling infrared detectors to 2K is very important to infrared astronomy as the nominal cosmic background radiation occurs at 3K. Thus Helium II has become a key enabling technology in many large science projects.
The Large Hadron Collider (LHC) recently commissioned at CERN operates its magnets at 1.9K to take advantage of the higher performance of NbTi superconductors at these temperatures.
Almost all the particle accelerators that use SCRF cavities operate entirely or in part at He II temperatures. These include the CEBAF accelerator at Jefferson Lab, the FLASH accelerator at DESY, Spallation Neutron Source (SNS) at Oak Ridge and the proposed International Linear Collider (ILC), Project X (Fermilab) and Facility for Rare Isotope Beams (MSU) machines.
He II has also been used in the very successful IRAS, COBE and WMAP space missions to map the universe in the infrared spectrum. All of these projects have also benefited from the enhanced heat transfer properties of He II.
Due to its technological usefulness, a great deal of effort has been invested over the years to develop the necessary equipment to reliably produce and use He II. This work includes the development of reliable cold compressors for large scale refrigeration systems and thermometry calibrated to operate at these lower temperatures. Studies of key engineering behaviors of He II including heat transfer, pressure drop and two-phase flow have also been undertaken. Today the use of Helium II as a coolant in large scale systems is well understood and supported by industry.
A very good discussion on the properties and behavior of He II may be found in Helium Cryogenics by S.W. Van Sciver, Plenum Press (1986). Recent examples of He II applications may be found in: “Exergy Analysis of the Cryogenic Helium Distribution System for the Large Hadron Collider (LHC),” S. Claudet et al, “Jefferson Lab 12 GeV Upgrade,” C. Rode, and “Herschel: An Overview of the Spacecraft,” C. Jewell et al. These last three are in the forthcoming Volume 55 of Advances in Cryogenic Engineering.
